The present invention relates to a coextrusion die and, more particularly, to a coextrusion die containing both a microlayer assembly and one or more distribution plates to produce coextruded films having both microlayers and thicker, conventional film layers.
Coextrusion is a technique for producing a multilayer plastic (polymeric) film by bringing two or more molten polymers together in a die, in which the polymers are formed into a generally tubular or planar shape, juxtaposed in layered form, and then pushed out of an annular- or slot-shaped opening in the die. Once outside of the die, the still-molten multilayer film is exposed to an environment having a temperature that is maintained below the melting point of the component polymeric layers of the film, which causes the layers to melt-bond together as they cool and solidify.
Multilayer films typically have a thickness in the range of 50-200 mils upon emergence from the die, but the films are generally stretched prior to final solidification in order to expand their surface area and reduce their final thickness to a range of about 0.5 to about 50 mils. Conventional multilayer films generally have 3-10 layers; prior to stretching and thinning, i.e., while still in the die, each such layer generally ranges from about 20-100 mils in thickness.
Microlayer extrusion is a technique for increasing the total number of layers in a multilayer film for a given film thickness, by reducing the thickness of the component layers of the film. Thus, while conventional film layers generally range from 20-100 mils inside the die (i.e., prior to stretching and thinning), microlayers generally have an ‘in-die’ thickness ranging from about 1-20 mils. In this manner, microlayered films may have far more than 10 layers, e.g., 20, 30, 40, 50, or more layers. Such microlayered films have been found to provide certain beneficial properties relative to conventional films composed of thicker layers that are fewer in number, e.g., improved mechanical properties such as superior flex cracking and puncture resistance.
For many applications, it is desirable to combine thicker, conventional layers with microlayers. Such thicker layers are often superior to microlayers for functions such as heat-sealing and abuse-resistance.
Unfortunately, it has proven to be difficult to combine the flow of thin layers, such as microlayers, with relatively thick layers in such a way that the physical integrity and independent properties of the thin layers are maintained. This is primarily the result of interfacial flow instabilities, which are encountered when microlayers are merged together with thicker layers in a die. Such interfacial flow instabilities are caused by the more powerful sheer forces of the thicker layers flowing against the microlayers, which result from the higher mass flow rate of the thicker layers relative to the microlayers. The resultant loss of the integrity and independent characteristics of the microlayers diminishes or even eradicates the beneficial properties thereof.
Accordingly, there is a need in the art for an improved die that permits microlayers to be combined with conventional, thicker layers in such a way that the integrity and independent properties of the microlayers are maintained.